Hybrid normal-reverse prism coupler for light-emittingdiode backlight systems

Shigeru Aoyama, Akihiro Funamoto, and Koichi Imanaka

For the first time to our knowledge, a hybrid normal-reverse prism coupler was formed on the bottomsurface of a light guide in a LED backlight system to achieve a thin, lightweight, LED backlight system.The hybrid prism coupler (HPC) simultaneously exhibits two functions: extraction of guided light fromthe light guide and focusing the radiated light from the light guide, corresponding to the optical functionsof the prism and diffusive sheets used in conventional LED backlight systems. Therefore, using a HPCeliminates the prism and diffusive sheets that have been indispensable optical elements in conventionalLED backlight systems, which consequently reduces the thickness of the LED backlight system by 40%compared with conventional systems. © 2006 Optical Society of America

OCIS codes: 230.0230, 230.5480, 130.3120.

1. Introduction

Mobile phones have noticeably undergone double-digit sequential growth over the past several yearsbecause they are an attractive information gatewayand offer portability. To achieve continued growth ofmobile phone use in our current multimedia society,mobile phones must offer high image quality and im-proved portability by achieving thin, lightweight com-ponents. In particular, backlight systems play animportant role for mobile phones from the perspectiveof high image quality control any time and any place aswell as the size of the mechanical structure.1–3 Toachieve high image quality, major efforts have focusedon improving brightness for backlight systems. Forexample, in current backlight systems, several types ofprism and diffusive sheets have been adopted to im-prove brightness based on focusing radiated light froma light guide. However, such optical sheets result in abacklight system that is 40% thicker than withoutoptical sheets, which decreases portability. A highlyscattering optical transmission polymer backlight sys-tem improved brightness, but it also employed prism

sheets.4 To date it has been difficult for backlightsystems to achieve simultaneous high brightness anda thin structure for portability without employingoptical sheets.

On the other hand, micro-optical devices are becom-ing standard in modern optical communication, mem-ory, and display systems because they are lightweight,can easily be mass produced, and are thin opticalsystems.5–7 In several types of micro-optical device,combined or hybrid micro-optical devices are attractivebecause they can satisfy the preceding requirementsby introducing optical multifunction capability.

Here we propose a hybrid normal-reverse prismcoupler for mobile phones that achieves a thin, light-weight LED backlight system. This hybrid prismcoupler (HPC), formed on the bottom of a light guide,simultaneously extracts the guided light in additionto focusing the radiated light from the light guide.Therefore, the proposed HPC is the first device thateliminates both the prism and the diffusive sheets,which have been indispensable optical elements inconventional LED backlight systems.

First we describe the structure and principle of thehybrid normal-reverse prism coupler. Then we dis-cuss the HPC’s arrangement on the light guide of theLED backlight system, primarily focusing on thepeak intensity and uniformity of the radiated light.Finally, experimental results of the prototype LEDbacklight system are discussed.

2. Design

A schematic view of the proposed HPC is illustratedin Fig. 1. The LED backlight system consists of a LED

The authors are with the Advanced Device Laboratory, Corpo-rate Research and Development Headquarters, Omron Corpora-tion, 9-1 Kizugawadai, Kizu-cho, Soraku-gun, Kyoto 619-0283,Japan. S. Aoyama’s e-mail address is shigeru_aoyama@omron.co.jp.

Received 9 November 2005; revised 23 March 2006; accepted 23March 2006; posted 24 March 2006 (Doc. ID 65844).

0003-6935/06/287273-06$15.00/0© 2006 Optical Society of America

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light source, a light guide, and a reflection sheet with-out prism or diffusive sheets. The LED light source isplaced on the corner edge of the light guide and con-sists of two LED bare chips. The light radiated fromthe LED light source is incident at the edge of the lightguide. The reflection sheet, located under the lightguide, reflects the transmitted light through the bot-tom of the light guide back to the upper side of the lightguide. HPCs are formed on the bottom surface of thelight guide, circularly arranged to center the LED lightsource. The HPC controls the light coupling efficiencybetween the guided and the radiated light as well asthe divergence of the radiated light.

A. Structure of the Hybrid Normal-Reverse Prism

The structure of the HPC must be modified to controlthe divergence of the radiated light. First, let us con-sider the behavior of the guided light. In the light-guide, guided light normally strikes the HPC becausethe concentric arrangement of the HPC centers theLED light source. Therefore, the light radiated fromthe light guide is extracted by the HPC, and thetransmitted light travels straight through the HPCin the radial direction plane. Therefore, it can beassumed that ray trace is only in the radial directionplane.

Ray traces for the hybrid prism and a conventionalprism coupler are illustrated in Fig. 2. The radiated

light divergence directivity in the tangential direc-tion becomes high due to the adoption of a point lightsource. On the other hand, the light radiated in theradial direction, as shown in Fig. 2(a), has a signifi-cantly larger divergence than in the tangential direc-tion, which corresponds to the order of the guidedlight mode, reducing light intensity. To improve lightintensity, focus in the radial direction is required.The incident light of the conventional prism couplercan be divided into two types: mode 1 is light incidentto the prism coupler that reflects from the bottomof the surface of the light guide within distance d1 atthe front of the prism coupler; mode 2 is incident lightto the prism coupler that satisfies the total internalreflection conditions at the surface of the prism cou-pler, except for mode 1. For both modes 1 and 2,guided light is transferred by the prism coupler to theradiated light from the light guide. The guided lightin mode 1 causes the radiated light to diverge. Thereverse prism coupler is placed in a region withlength d1. The angular spectra of the incident light tothe reverse prism coupler are transferred to the lowerangular spectra, which correspond to the mode 2 re-gion, by reflecting the incident light at the reverseprism’s surface. The transferred light is extracted bythe normal prism coupler. It should be noted that theHPC allows simultaneous functions to occur: concen-tration of the radiated light from the light guide andextraction of the guided light to the radiated light.

Figure 3 shows the calculated radiation angle ofthe peak light intensity variation resulting from thebase angle � variation of the reverse prism. For thisplot, the radiation light angular profile from the LEDlight source is assumed to be Lambertian, the baseangle of the normal prism � is 45°, d1 is 6 �m, d2 is2.5 �m, and the light guide’s refractive index is 1.5.The base angle of the reverse prism is set at 20°, sothat the angular spectra of the peak light intensityextracted by the reverse prism coupler coincides withthat from the normal prism coupler.

Figure 4 shows the dependence of the full width athalf-maximum (FWHM) of the radiated light inten-sity from the light guide on the ratio of the baselength between the normal and the reverse prisms.For this plot, the base angle of normal prism � is 45°,and the base angle of the reverse prism is 20°. From

Fig. 2. Cross-sectional illustration of (a) a conventional prismcoupler and (b) a HPC.

Fig. 1. Illustration of a hybrid normal-reverse prism coupler for aLED backlight system.

Fig. 3. Calculated results for the peak intensity of radiated lightfrom a light guide versus the base angle of a normal prism.

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these results, the ratio of the base length is set at 1.2so that the radiated light divergence becomes mini-mum. According to the above design, the FWHM an-gular spectra of light intensity radiated in the radialdirection for the HPC can be designed one-halfsmaller than those for a conventional normal prism.

B. Arrangement of the Hybrid Normal-Reverse Prism

Light coupling efficiency between the guided and theextracted light can be controlled by the density vari-ation arrangement of the HPC as a function of posi-tion from the light source. Arrangement density isvaried by modification of the HPC’s length. The ar-rangement of the hybrid prism is designed withconsideration of radiated light uniformity. Diffusivesheets improve the uniformity of radiated light inten-sity in conventional backlight systems. The HPCsare concentrically arranged, centering the LED lightsource; the main feature is that the guided light trav-els straight in the light guide because the guided lightnormally strikes the HPC. Therefore, the arrange-ment of the HPCs can be designed only if we take intoconsideration the radial direction. Arrangement ofthe density variation, which is required to achieve uni-formity of the radiated light, is determined by theHPC’s length in the tangential direction. To achieveuniform intensity, light coupling efficiency is designedto increase in proportion to the distance from the LEDlight source.

Figure 5 is a schematic illustration of the deriva-tion of light coupling efficiency.8 Light with power P0strikes in region �� from the light source. L denotesthe distance from the light source to the point atwhich the guided light power becomes zero; radiatedlight intensity in the r�r�� region at distance r fromthe light source is q, which is assumed to be constantin the r�r�� region. Incident light power P0 is repre-sented by

P0 ��0

L

qrdr. (1)

Radiated light intensity q is

q � �2P0�L2�. (2)

Radiated light power at distance r from the lightsource is given by

Q�r� � �2P0�L2�rdr. (3)

Guided light power S�r� is given by

S�r� � P0 ��0

r

qrdr, (4)

S�r� � P0�1 �r2

L2�. (5)

Light coupling efficiency ��r� is

��r� � �Q�r��S�r��, (6)

��r� � �2r��L2 � r2��dr. (7)

Coupling efficiency must be varied according to Eq.(7) so that the radiated light power from the lightguide becomes constant at any position. The varia-tion of coupling efficiency corresponds to the varia-tion of the transverse length of the hybrid prismarray. Here we consider that the coupling efficiency ofunit length �� in the radial direction at distance r isgiven by 2r��L2 � r2�. The transverse length of theHPC corresponding to required coupling efficiency �=is numerically obtained based on the ray trace. Fig-ure 6 is a plot of the numerical results for the relationbetween the transverse length of the HPC and thecoupling efficiency. The light guide is 0.65 mm thick,the reflectivity of the reflection sheet is 91%, the re-fractive index is 1.5, the pitch of the HPC is set at

Fig. 5. Derivation of light coupling efficiency. Light with power P0

strikes in region r�r�� from the light source. L denotes the distancefrom the light source to the point at which the guided light powerbecomes zero. Radiated light intensity in the r�r�� region at dis-tance r from the light source is q, which is assumed to be constantin the r�r�� region.

Fig. 4. Calculated results for dependence of the FWHM of radi-ated light intensity from the light guide on the ratio of the baselength between normal and reverse prisms.

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50 �m, and the transverse length varies from 2 to80 �m. The ray number is taken to be 20,000 in raytrace calculations using TracePro from the LambdaResearch Corporation. In this calculation we considerFresnel reflection. So the transverse length of theHPC at any position can be chosen from the resultsobtained from the relation between the coupling effi-ciency and the transverse length of the hybrid prism.

3. Experiments

A. Device Fabrication

Figure 7 shows the fabrication process of the HPC.The resist sensitivity curve, which is a characteristicof residual resist thickness dependence on the elec-tron dose, must be obtained before fabricating theHPC’s desired structure. For this purpose, an 8 in.(20 cm) silicon wafer was spin coated with positiveresist (OEBR-1000, Tokyo-Ohka Kogyo, Kawasaki,Japan) to a thickness of 5 �m and then prebaked in aN2 atmosphere oven at 170 °C for 20 min. The bakedresist layer was coated with a 20 nm Au layer. A testpattern was fabricated; the electron dose varied from

10 to 100 �C�cm2 in 5 �C�cm2 steps. Exposure wasperformed with an Elionix (Tokyo, Japan) ELS3300electron-beam machine using an acceleration voltage

Fig. 6. Numerical results for the relation between the transverselength of the HPC and the coupling efficiency.

Fig. 7. Process flow diagram for the HPC.

Fig. 8. SEM photographs showing the top view arrangement of areplicated HPC: (a) 23 mm and (b) 30 mm from the LED lightsource.

Fig. 9. SEM photographs of a replicated HPC: (a) cross-sectionalview and (b) top view.

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of 50 kV and a beam step size of 100 nm. After expo-sure, the Au layer was dissolved in a KI solution, andthe exposed pattern was developed in a solution ofmethyl isobutyl ketone and isopropanol (1:2) for 60 s.Development was stopped by holding the sample inwater for 15 s. Residual resist thickness was foundlinearly varied at an electron dose from 10 to 50�C�cm2. We determined the design’s electron dose byapplying the linear region of the electron dose to thedesired cross-sectional profile of the HPC, as shownin Fig. 7. Exposure of the HPC was performed withthe electron dose determined by the above process.The stamper was fabricated by an electroformingprocess. The patterned resist layer had to be coatedwith a 50 nm thick conductive nickel layer by thesputter method. The master was placed in an elec-troforming bath using an electric current of 5 mA for5 h to produce the stamper. A typical thickness of thefabricated stamper was 250 �m. We performed rep-lication by hot embossing a replicated 0.65 mm thickpoly(methyl methacrylate) substrate. The substratewas placed on the bottom stage, and the stamper wasplaced on the upper stage. The upper stage was thenheated above the glass transition temperature ofpoly(methyl methacrylate) under 10 N�m2 pressure.After the pressure was released and the stage wascooled, the stamper and the replicated substrate wereseparated.

B. Results and Discussion

Scanning electron microscope (SEM) photographs ofthe top and cross-sectional views of the replicatedHPC are shown in Fig. 8 and 9, respectively. The baseangle and length of the reverse prism are 20° and8.0 �m, respectively. The base angle of a normalprism is approximately 47°. It was determined that afairly well-controlled profile could be obtained. SEMphotography shows that the arrangement density

adjacent to the LED light source is low and in-creases in proportion to the distance from the LEDlight source.

Figures 10(a) and 10(b) show the light intensityprofiles of the LED backlight systems obtained withhybrid and conventional prism couplers, respectively.The light intensity profile exhibits approximate sym-metry that is due to a focusing effect in the radialdirection for the HPC. The FWHM of the radiatedlight intensity in the radial and tangential directionsfor the HPC is 38° and 23°, respectively, and 94° and17° for a conventional prism coupler. The peak lightintensity obtained for the HPC is 3400 cd�m2, whichis two times larger than for a conventional prism.

4. Conclusion

To achieve a thin, lightweight LED backlight system,a hybrid normal-reverse prism coupler was presentedfor a LED backlight system. The HPC can simulta-neously perform the basic functions of focusing radi-ated light from a light guide in addition to extractinglight from guided light. The operating principle ofboth the focusing and the extraction functions hasbeen experimentally demonstrated. The resultantlight intensity profile for this LED backlight systemwas confirmed to be symmetrical because of the ra-dial direction focusing effect of the HPC; it was asym-metrical for a conventional LED backlight. Such aHPC promises to advance the development of thin,lightweight, LED backlight display systems that canprovide high quality image displays.

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Fig. 10. Experimental results for the light intensity profile of a LED backlight system using (a) a HPC and (b) a conventional prismcoupler.

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